Of the various electrostatic printing methods, electrophotography has dominated high resolution monochrome printing for several decades. The electrophotographic process includes uniformly coating a photoconductive surface with charge, selectively exposing the charged surface with light to form a latent image, developing the latent image by causing charged toner particles to come in contact with it, transferring the image to a receiving sheet, and fixing the image. This printing method has produced high quality printing and has been refined to effectively service a broad range of printing applications. However, it is mechanically complex, requires precision optical components, and has proven difficult to adapt to color printing.
Direct Electrostatic Printing (DEP) can be simpler than electrophotographic printing. In U.S. Pat. No. 3,689,935, Pressman et al. disclose a DEP device in which toner is deposited directly through apertures onto a plain paper substrate in image configuration. This method has been improved by Schmidlin in U.S. Pat. No. 4,912,489 issued Mar. 27, 1990 in which a control voltage as low as 100 V is sufficient to modulate the flow of toner through the apertures. The Schmidlin device employs a traveling wave conveyor to present toner to the print head apertures. U.S. Pat. No. 3,113,042 issued to Hall in 1963 describes a magnetic toner conveyor for use as a developer unit in a xerographic printer. Magnetic powder is transported from a toner reservoir to develop a latent image on a xerographic plate or drum. The conveyor has a linear structure with multiple phases driven by current sources to magnetically convey the toner. U.S. Pat. No. 3,778,678 issued to Masuda in 1974 describes a voltage traveling wave device for moving particles along a tubular duct. The electrodes are spirally wound along the outer surface and are connected to a three-phase alternating current source of 5-10 kV, which repels the particles from the inner surface and propels them along the tube. In U.S. Pat. No. 3,801,869, Masuda also describes a grid of planar spaced-apart electrodes covering the wall of a paint booth for transporting paint particles for the purpose of removing paint from the wall. U.S. Pat. No. 4,527,884 issued to Nusser in 1985 describes a device for applying toner to an electrostatic charge image carried on an information carrier. The apparatus employs a traveling wave conveyor to transport the toner in the form of an aerosol to the information carrier where a development gap is created between the surface of the traveling wave conveyor and the information carrier. Toner is transferred across the gap to the information carrier to develop the image. U.S. Pat. No. 4,568 issued to Hosoya et al. in 1986 describes the use of a three-phase traveling wave conveyor to create a toner fog at the surface of a developer carrier. U.S. Pat. No. 4,647,179 issued to Schmidlin describes apparatus employing traveling wave transport of the toner. In 1989, Melcher et al. (J. R. Melcher, E. P. Warren, and R. H. Kotwal, "Theory for pure-traveling wave boundary-guided transport of tribo-electrified particles", Particle Sci. Technol., vol. 7, no. 1, 1989) (J. R. Melcher et al, "Traveling-wave delivery of single component developer", IEEE Trans. Industry Applications, vol. 25, no. 5, pp. 956-961, September /October 1989) provided additional understanding of the modes of transport that are achievable with voltage traveling waves. In 1990 Schmidlin (Fred W. Schmidlin, "A new nonlevitated mode of traveling wave toner transport", paper IUSD 89-62, approved by the Electrostatic Process Committee of the IEEE Industrial Applications Society for presentation at the 1989 Industry Applications Society annual meeting, Pittsburgh, Pa. Oct. 2-7, 1989, and released for publication Dec. 6, 1990) published a description of a charged toner conveyor wherein the particles are not levitated and are carried synchronously with the traveling wave.
The aforementioned patents and other publications on traveling wave toner conveyors address their use for conveying toner, but not for imaging. In U.S. Pat. Nos. 5,153,617, 5,287,127, and 5,400,062 Salmon has extended the use of traveling wave devices to imaging of toner using a variety of direct writing heads. The writing heads are typically in the form of flat panels or flex circuits that extend across the printing width, and they generally provide an independent traveling wave channel for each pixel to be printed. This direct imaging technology has become known in the industry as Digital Packet Printing, or DPP.
The present invention builds on the concept of direct writing heads employing voltage traveling waves. It describes an electrostatic toner loading apparatus and method to create a convenient source of charged toner at the surface of a planar member. The loading method causes toner particles mixed with air to periodically sweep the planar surface. The particles are available for imaging at the surface of the planar member. A feature of this approach is that the thin film circuits on the planar surface that create the toner loading action can co-exist and co-function with circuits on the same planar surface that are used to image the particles. Further, it will be shown that the imaging circuits can form images by either additive or subtractive means, providing a broad range of printer design solutions.
Another improvement in this application is the provision of barrier electrodes to channel the toner flow into pixel-wide columns corresponding to the pixels across the image formed on a receiving sheet. Toner flow in each channel may be individually modulated to further create pixels in the process direction and gray-levels within each pixel. Once a modulated flow of toner packets or pixels is created in each channel by additive or subtractive means as described herein, the barrier electrodes help to keep toner packets segregated into pixel-wide columns and prevent crosstalk between adjacent channels. In summary, the barrier electrodes channelize toner transport on a surface, and minimize cross-talk between the channels.
A further improvement described in this patent application is the use of Cyclotene as a topcoat layer for the imaging structures. Cyclotene has the desirable property of being essentially triboelectrically neutral to charged toner touching events. As toner is transported along a traveling wave channel, some of the particles touch or slide against the supporting surface. If touching or sliding events changed the particle charge, it would be difficult to repeatably control the charge on toner particles and therefore the electrostatic force. Conversely, if the touching events are charge neutral, the charge to mass ratio of the particles remains relatively constant, and the fields asserted by the voltage traveling waves and the imaging electrodes control and move the toner in consistent and predictable ways.
In 1985 Hosoya et al. describe a xerographic developer unit for single component non-magnetic toner employing a regulating plate pressed against a donor roll to meter the toner into a thin layer and to charge it triboelectrically. They describe its use in non-contact development of a photoconductive drum. The effects of surface roughness, plate pressure, and bias voltage are described.
A similar device serves as a loading apparatus. For the current invention, what is needed is well charged toner that can be presented to a writing head. There are multiple triboelectric charging units developed by others that meet these requirements, as well as some that charge the toner using corona fields.